In recent years, Toll-like receptors (TLRs) have emerged as critical recognition elements of innate immunity, both for induction of inflammation at the site of an infection and for induction of an adaptive immune response. These receptors are expressed on the three major types of immune cells in many tissues, immature dendritic cells, tissue macrophages, and mast cells, as well as on several other types of cells. In the proposed project, we shall define which type of cell is responsible for mediating several TLR-based immune responses, systemically and in the airways. In these studies, we shall take advantage of a conditional allele we have engineered into the mouse germ line for the key TLR signaling adaptor molecule MyD88. This allows us to delete the gene encoding MyD88 specifically in particular cell types including dendritic cells, macrophages and neutrophils, B cells, and T cells. In Aim 1, we shall determine the role of dendritic cells in a mouse model of allergic asthma in which antigen + TLR ligands are introduced via the airways. In Aim 2, we shall analyze the immune response of mice to systemic or airway exposure to zymosan, a yeast cell wall preparation that is composed of chitin, ligands for TLR2 and ligands for C-type lectin receptors (dectin-1, etc.). We shall dissect the roles that these different innate immune ligands play in directing the nature of the immune response between Th1, Th2 and Th17. The contributions of particular cell types will also be determined. Finally, in Aim 3, we'll determine the cell type-specific roles of MyD88 in host defense to systemic infection by the fungal pathogen, Candida albicans. Lay Language Summary: The proposed studies will determine which immune cells in tissues are responsible for initiating immune responses to inhaled antigens as a model of allergic asthma, to infections with yeasts and molds. This will be accomplished by the use of genetically modified mice, in which key immune cell types are unable to recognize the presence of fungal cell walls. These studies will be useful for improving vaccination strategies and for developing novel strategies to block inflammation for patients with inflammatory diseases.